Field of the Invention
This invention relates generally to a spiral system for providing cooling air to a turbine casing in a gas turbine engine and, more particularly, to a spiral system that provides cooling air to a turbine casing in a gas turbine engine, where the system includes spirally configured fins mounted to an inside surface of the casing within a casing plenum to provide a longer path for the airflow to provide more cooling.
Discussion of the Related Art
The world's energy needs continue to rise which provides a demand for reliable, affordable, efficient and environmentally-compatible power generation. A gas turbine engine is one known machine that provides efficient power, and often has application for an electric generator in a power plant, or engines in an aircraft or a ship. A typical gas turbine engine includes a compressor section, a combustion section and a turbine section. The compressor section provides a compressed airflow to the combustion section where the air is mixed with a fuel, such as natural gas. The combustion section includes a plurality of circumferentially disposed combustors that receive the fuel to be mixed with the air and ignited to generate a working gas. The working gas expands through the turbine section and is directed across turbine blades therein by associated vanes. As the working gas passes through the turbine section, it causes the blades to rotate, which in turn causes a shaft to rotate, thereby providing mechanical work.
The turbine section of a typical gas turbine engine will include a plurality of rows of circumferentially disposed blades, such as four rows of blades, where the working gas is directed by a row of vanes across the blades from one stage of the blades to the next stage of the blades. It is generally desirable that the outer tip of the rotating blades be as close as possible to the static casing surrounding the blades, referred to in the art as tip clearance, so that a maximum amount of the working gas flows around the blades instead of flowing between the blades and the casing, which does not contribute to rotation of the blades, to provide improved blade performance. As the temperature of the engine goes up and down, the blades and casings expand and contract accordingly, which changes the tip clearance. Also, the centrifugal force from rotation of the blades causes the length of the blades to increase, which reduces the tip clearance. It is generally the tip clearance of the blades at system steady state operation that determines the performance of the blades and therefore of the engine. On the other hand, the tip clearances are also crucial in ensuring that the blades do not rub with static hardware during the startup and shutdown procedures of the engine because of different thermo-mechanical expansions and/or contractions of the blades and casings. Thus, tip clearances are set appropriately in an engine so as to derive the best performance and prevent tip rubbing.
Centering the turbine rotor within the gas turbine stationary casing is a challenge that affects the turbine blade tip clearances, and thus can negatively impact turbine performance. Lack of rotor center line positioning control during cold engine build and the subsequent control of the center line excursions under transient thermal and mechanical conditions forces the turbine blade to stator clearances to be set sufficiently large to prevent excessive rubbing or clashing, where these increased clearances result in loss of turbine efficiency and reduction in output power.
At the output of the turbine section, the working gas is passed through an exhaust gas diffuser that recovers the dynamic head of the exhaust gas for optimal performance of the turbine section. The exhausted gas, which is still very hot, is often times directed to other systems that may benefit from the available heat until the working gas is eventually exhausted to the environment or otherwise. For example, the hot exhaust gas at the output of the gas turbine engine may be used to boil water for a steam turbine engine, which also generates power in, for example, a combined cycle plant, well known to those skilled in the art. The configuration of the exhaust gas diffuser at the output of the gas turbine engine is important for the performance of the gas turbine blades because the exhaust gas diffuser partially blocks the gas flow from the turbine section.
The turbine rotor rotates on turbine bearings within a bearing housing. In one gas turbine engine design, a star member supports the turbine rotor downstream of the turbine section, where the star member includes a plurality of struts extending from a central annular portion and having ends that are secured to an aft flange of the turbine casing, and where the bearing housing is positioned within the annular portion. Hot exhaust gas flow, for example, in excess of 1200° F., from the turbine section of the engine flows through the star member around the struts.
Cooling air is pumped through a configuration of cooling channels in the turbine section so that the cooling air cools the outer turbine casing, and usually flows through channels in the star member struts to the bearing housing to cool the bearings therein to a desired operating temperature. Various cooling flow configurations for the design of the star support member and other components cause variations in temperature between different areas of the turbine casing and different struts in the star member. These temperature differentials cause deformities in the outer turbine casing as a result of the struts being secured at different locations to the aft flange of the casing that causes the casing to become out of round, which affects the tip clearance of the turbine blades. More particularly, because the struts are bolted to an aft flange of the turbine casing with heavy bolts, the different temperature struts provide different pressures to the casing causing the deformation of the casing, which changes the centering position of the turbine rotor.